In the processing of semiconductor wafers, the wafers are frequently heated to elevated temperatures for processing in vacuum deposition devices, such as physical vapor deposition chambers. Thermal treatment of the wafers is sometimes a necessary or incidental component of a coating or etching process and at other times is the essence of the process itself, such as in rapid thermal processing, annealing or degassing. The temperature of the wafer is critical to producing a high quality, reliable wafer. If the temperature goes awry, either high or low, then the material or metal properties and microstructure of the interconnect changes, which has a direct impact on the reliability of the material or metal films that have been deposited, and in the worse case, it may result in a device failure.
In attempts to maintain control over the wafer temperature within the vacuum deposition device, various temperature schemes have been employed in these deposition devices. Thermocouples, for example, attached to or held in contact with the wafer, or mounted in the wafer support have been used. However, these devices are known to be slow to respond to temperature changes and often introduce a source of wafer contamination. Moreover, the wafers are usually moving in and out of the deposition device in a relatively short time frame, which can range from 10 seconds to 3 minutes. Attaching thermocouples directly to the wafer slows the transfer process, and thus the overall process, down to undesirable levels. Additionally, to maintain the vacuum required for high quality deposition of the materials, vacuum feed throughs for the thermocouples should be present. Thus, the equipment has to be modified with the vacuum feed throughs, which makes the process extremely cumbersome. Such direct thermocouple contact with the wafer can also require very long pump down times such that it takes hours to do one temperature measurement. Therefore, the process for periodically measuring wafer temperature becomes impracticable.
Pyrometers have also been employed to measure wafer temperature. While such pyrometer techniques avoid direct contact with the wafer being processed, they have the disadvantage of being sensitive to the emissivity of the material of which the wafer or coatings added to it are made; emissivity is also subject to change during processing.
Optical methods for indirectly measuring the temperature of an article by measuring its thermal expansion have also been used. These optical methods provide an advantage of being related only to the coefficient of thermal expansion of the material that forms a structural core of the wafer, which is constant and can be reliably determined. However, such optical methods also suffer from disadvantages. For example, some optical methods include the formation of images of the article that require equipment, such as optical sensors, to be placed close to the article where their accuracy can be affected by the process within the chamber. Such methods are in practice used off-line, and where so used, atmospheric refraction can also detract from the accuracy of the measurement process. Furthermore, where articles such as semiconductor wafers are mounted on a support within a sealed chamber for thermal processing, misalignment or distortion of the support may alter the wafer position and render optical temperature determination methods unreliable. Yet other optical methods involve specially designed equipment and methods to over come these problems, which is not completely desirable. The reason for this undesirability is that the specially designed equipment and methods associated with that equipment must be used, which precludes the use of conventional physical vapor deposition equipment and methods in the semiconductor wafer manufacturing process.
In addition, the deposition equipment manufacturer provides curves that relate heater block temperatures to the semiconductor wafer temperature. These curves, however, can be unreliable with respect to different deposition devices because each deposition device may vary with respect to another, thereby causing inaccuracies in the application of the standardized curves. Moreover, the physical properties of wafers may vary from one to another, which may also make cause additional inaccuracies in the application of the standardized curves.
Accordingly, what is needed in the art is a way of accurately determining the temperature of a semiconductor wafer while it is in a conventional deposition device and calibrating the device based on the temperature of the heater within the deposition device as it relates to the wafer temperature. The method of the present invention addresses these needs.